Humidifiers are sometimes utilized in HVAC systems in order to provide humidified air for the system and the occupants of the building. Particularly in northern climates which require heated air pumped through the building, it is sometimes necessary to humidify the air in order to create a pleasant work environment for the occupants of the building.
Typical humidifiers utilize a large tank or container, sometimes called an evaporation chamber, (ranging in size from approximately 9 inches in diameter (or smaller) to approximately 34 inches on a side for a square humidifier chamber) which holds water. Heating elements are commonly disposed at positions such that the elements are under water. When the HVAC system requests humidified air (based upon a user controlled humidistat signal or a humidifier ON control signal), the heating element is turned on, the water boils and humidified air is supplied to the HVAC duct system in the building. The humidified air leaves the humidifier when the pressure in the humidifier exceeds the static P.S.I. in the HVAC duct work. Sometimes, humidified air is pulled from the humidifier via a fan box. At other times, the humidified air is injected into the system by the pressure differential between the humidifier and the HVAC duct system.
Water is customarily supplied to the humidifier as pressurized city water. Other water supply systems may be utilized such as pump driven systems. Accordingly, when the heater or heating coil raises the temperature of the water to its boiling point, the water, or portion thereof, changes to steam and the steam, after being mixed with air in the HVAC duct system, becomes humidified air. Since impurities commonly found in the water supply build up in the humidifier, due to the continued evaporation of H.sub.2 O molecules, the water captured by and retained in the humidifier becomes increasingly contaminated with impurities, which may be minerals and salts, initially found in trace amounts in the water supply. The increasing concentration of impurities generates foam when the water in the humidifier is boiled.
Since the H.sub.2 O molecules are converted into steam, the water level in the humidifier continually decreases. At some point in time, the water level in the humidifier drops to an unacceptable level which exposes a significant portion of the heating elements. Heater elements operate at about 60-70 WATTS per square inch. When this occurs, the heating elements primarily heat the air in the humidifier box or container rather than the water. Hence, the temperature of the air in the humidifier unit increases and there is an increased risk of fire or other thermal damage to the HVAC system, the associated control system and the building. When the heater transfers most of its thermal energy to the water (the water acts as a heat sink), the temperature in the humidifier is about 100.degree. C., plus or minus 2 or 3 degrees. Thermal conditions in excess of 104 degrees C. are not acceptable.
In order to avoid this thermal run-away problem, prior art or pre-existing humidifier control systems utilize conductivity sensors to determine the level of water in the humidifier. Conductivity sensors are triggered when the resistance to ground is lower than a predetermined threshold and are designed to be activated by water in the humidifier. However, with the creation of significant amounts of foam, the foam is an effective conductor of the electricity. Since the foam is conductive, the humidifier control system may detect "false high levels" of water in the humidifier system. If a false high level of water is detected due to foam and the water drops below the heater elements, the system will experience significant thermal damage.
In order to avoid these problems, prior art or pre-existing humidifier control systems drain the humidifiers of water to remove water with a high concentration of impurities based upon a predetermined drain cycle time. For example, in an area having water with a high level of impurities, a large humidifier may be drained and refilled with fresh water every four hours. The same humidifier, when supplied with purer water, may be drained every six hours. The difference between the two systems is the level of impurities in the water. It is known that impurities cause foam in humidifiers. Further evaporation and decreasing water levels increase the impurity level of the water remaining in the humidifier. This, in turn, generates greater amounts of foam, increasing the risk of false high water readings from conductive sensors and potentially dangerous thermal conditions.
However, since stable thermal conditions are required, the fill and drain cycles are set for shorter time periods than necessary. As a result, humidifier systems using these "closed" or "fixed" drain cycles utilize a considerable amount of energy. In addition to flushing and draining the water from the humidifier, the newly added water must be reheated to the boiling point (100.degree. C.) in order to provide humidified air to the HVAC system.
In addition to the foregoing, the prior art systems could not detect large amounts of foam near the top air flow ports of the humidifier. Under certain conditions, the foam sometimes leaves the humidifier with the humidified air and is launched into other parts of the HVAC system.
Mechanical systems which detect the water level utilize floats and lever arms are also known. These mechanical systems are subject to corrosion and mechanical failures. The thermal cycling of the humidifier also adversely effects mechanical sensing systems. Further, mechanical systems do not detect the level of foam.
If the humidifier system utilizes pure or demineralized water, conductive sensors do not work because pure water is not electrically conductive. Accordingly, mechanical water level sensors must be used and the humidifier control systems must be altered to accept these inputs.
Some prior art or pre-existing systems utilize bimetallic temperature sensors mounted on the humidifier casing to detect a thermal run-away condition. These temperature sensors are not commonly disposed in the humidifier because the sensors are adversely effected by the water, heat and impurity levels in the humidifier.